The molybdenum coordination sites of the molybdoenzymes appear to fall into two classes, depending on the nature of the molybdenum-containing cofactor: the nitrogenases possess molybdenum bound to some type of Mo, Fe, S cluster, while it appears reasonably certain that Mo(VI), (V), and (IV)-oxo species are involved at the active sites of the molybdenum oxidases and nitrate reductase. In all cases, the molybdenum is coordinated to some type of sulfur-donor ligand. The major objective of the research is to explore the chemical and structural possibilities of molybdenum-sulfur complexes and to develop the electrochemical and reactivity trends exhibited by these species. The work in progress is concerned with the synthesis and complete characterization of complexes of Mo(VI), (V) and (IV) with polydentate ligands containing mercapto- and/or sulfido-donor groups. A recent extension of this investigation involves the reactions of these species with hydrazines and diazenes and protonation studies of the resultant hydrazido-complexes. Similar investigations are being pursued with Mo-Fe dimers and higher-order clusters of novel composition.